Global Navigation Satellite System (GNSS) receivers determine position by computing the relative time of arrival (TOA) of ranging signals that are continually transmitted from a plurality of GNSS satellites orbiting the earth and/or pseudolites (collectively referred to herein as “GNSS sources”). GNSS sources transmit satellite information such as timing and ephemeris data in addition to ranging signals. As described herein, GNSS sources can include the United States Global Positioning System (GPS), the Russian Glonass system, the European Galileo system, any system that uses satellites from a combination of satellite systems, or any satellite system developed in the future (collectively called as “SPS” or “Satellite Positioning System”). Furthermore, some position determination systems utilize pseudolites or a combination of satellites and pseudolites. Pseudolites are ground-based transmitters that broadcast a ranging code, such as a PN code (similar to a GPS or CDMA cellular signal), modulated on a carrier signal which may be synchronized with time provided by an SPS. Pseudolites are useful in situations where SPS signals from an orbiting satellite might be unavailable, such as in tunnels, mines, buildings, urban canyons or other enclosed areas. The term “satellite”, as used herein, is intended to include pseudolites, equivalents of pseudolites, and possibly others. The term “SPS signals,” as used herein, is intended to include SPS-like signals from pseudolites or equivalents of pseudolites. Additionally, other non-GNSS sources, such as one that is used for the Advanced Forward. Link Trilateration (AFLT) technique, may be used. Herein, GNSS sources and non-GNSS sources are collectively referred to as ranging sources.
Ranging Receivers often use omni-directional antennas to capture signals from all sources in view. Each channel of the ranging receiver is designed to be selective for a particular desired signal. The lack of spatial discrimination inherent in omni-directional antennas makes the ranging receiver susceptible to receiving multiple, delayed copies of the same desired signal, which causes corruption of the received signal. This type of signal corruption is known as multipath interference. In particular, multipath interference may cause errors in position determination or timing by the ranging receiver. Such multipath errors can also be experienced when using more traditional, optimized antennas with upward facing gain patterns.
Multipath interference is generally composed of a superposition of multiple copies of the same signal (e.g., the line of sight (LOS) signal and one or more reflected copies of the LOS signal, or an earliest reflected signal with one or more later delayed signals). In general, both LOS and reflected signals are part of the received composite signal. The delayed copies may comprise signal copies separated by various time delays. Time delay in CDMA systems is typically measured in units of chips, where one chip is the time duration of pseudonoise (PN) code elements (about a microsecond for the GPS course acquisition or C/A code).
Ranging receivers typically discriminate among multiple spread spectrum signals in the received composite signal through correlation processing. In correlation processing, the received composite signal is correlated with a reference signal matched to the desired signal to obtain a correlation function. Ideally, there is one distinguishable peak in the correlation function which indicates the presence and time delay of the desired signal. However, in the presence of multipath interference, there will be several multipath components with different peaks in the correlation function.
A typical ranging receiver can distinguish multipath components separated by some predetermined quantity of chips. For some systems, it can distinguish multipath components separated by at least 1.5 chips. Current multipath mitigation algorithms do not work well for short multipath scenarios. For example, some multipath mitigation algorithms do not work well where the short multipath components are typically separated by less than 1.5 chips. In addition, other algorithms used for multipath mitigation may have excessive computational burden.